1. Field of the Invention
The present invention relates to a three-dimensional structural member having enhanced load bearing capacity per unit mass. More particularly, the present invention relates to a structural member having a plurality of helical components wrapped around a longitudinal axis where the components have straight segments rigidly connected end to end.
2. Prior Art
The pursuit of structurally efficient structures in the civil, mechanical, and aerospace arenas is an ongoing quest. An efficient truss structure is one that has a high strength to weight ratio and/or a high stiffness to weight ratio. An efficient truss structure can also be described as one that is relatively inexpensive, easy to fabricate and assemble, and does not waste material.
Trusses are typically stationary, fully constrained structures designed to support loads. They consist of straight members connected at joints at the end of each member. The members are two-force members with forces directed along the member. Two-force members can only produce axial forces such as tension and compression forces in the member. Trusses are often used in the construction of bridges and buildings. Trusses are designed to carry loads which act in the plane of the truss. Therefore, trusses are often treated, and analyzed, as two-dimensional structures. The simplest two-dimensional truss consists of three members joined at their ends to form a triangle. By consecutively adding two members to the simple structure and a new joint, larger structures may be obtained.
The simplest three-dimensional truss consists of six members joined at their ends to form a tetrahedron. By consecutively adding three members to the tetrahedron and a new joint, larger structures may be obtained. This three dimensional structure is known as a space truss.
Frames, as opposed to trusses, are also typically stationary, fully constrained structures, but have at least one multi-force member with a force that is not directed along the member. Machines are structures containing moving parts and are designed to transmit and modify forces. Machines, like frames, contain at least one multi-force member. A multi-force member can produce not only tension and compression forces, but shear and bending as well.
Traditional structural designs have been limited to one or two-dimensional analyses resisting a single load type. For example, I-beams are optimized to resist bending and tubes are optimized to resist torsion. Limiting the design analysis to two dimensions simplifies the design process but neglects combined loading. Three-dimensional analysis is difficult because of the difficulty in conceptualizing and calculating three-dimensional loads and structures. In reality, many structures must be able to resist multiple loadings. Computers are now being utilized to model more complex structures.
Advanced composite structures have been used in many types of applications in the last 20 years. A typical advanced composite consists of a matrix reinforced with continuous high-strength, high-stiffness oriented fibers. The fibers can be oriented so as to obtain advantageous strengths and stiffness in desired directions and planes. A properly designed composite structure has several advantages over similar metal structures. The composite may have a significantly higher strength-to-weight and stiffness-to-weight ratios, thus resulting in lighter structures. Methods of fabrication, such as filament winding, have been used to create a structure, such as a tank or column much faster than one could be fabricated from metal. A composite can typically replace several metal comoponents due to advantages in manufacturing flexibility.
U.S. Pat. No. 4,137,354, issued Jan. 30, 1979, to Mayes et al. discloses a cylindrical "iso-grid" structure having a repeated isometric triangle formed by winding fibers axially and helically. The grid, however, is tubular instead of flat or straight. In other words, the members are curved. This reduces the buckling strength of the members as compared to a straight member.
Therefore, it would be advantageous to develop a structural member having enhanced load bearing capacity per unit mass and capable of withstanding multiple loadings.